Light emitting diodes using a double heterostructure AlGaInP have been demonstrated in recent years. A typical double heterostructure AlGaInP device has a GaAs n-type absorbing substrate on which several epitaxial layers are grown to form the light emitting device. An n-type confining layer of AlGaInP is first grown on the GaAs substrate. An active layer of undoped AlGaInP is then grown on the confining layer. The next layer is a top confining layer of p-type AlGaInP. The efficiency of such a light emitting device depends on the current spread in the top layer. Because of the high resistivity in the top p-type AlGaInP layer, the spread of current is generally small. Such a layer is typically very resistive and the light output is relatively low. Increase of the thickness in the top confining layer can widen the current spread and improve the efficiency of the light output. Nevertheless, it has been difficult to grow a thick AlGaInP layer.
Several techniques have been presented to improve the efficiency of double heterostructure AlGaInP light emitting devices. One technique involves forming a current blocking layer of high band gap and more electrically conductive material, such as GaAlAs, with various shapes of current blocking regions and structures above the top p-type AlGaInP layer to increase the current density. This technique has been known to be suitable for relatively long wavelength light such as red or orange. However, it does not work well in the shorter wavelength range, such as green and yellow. Another technique uses a lattice mismatched GaP window layer on top of the upper AlGaInP confining layer. The technique requires growing a fairly thick layer of GaP above the AlGaInP layer. Threading dislocations and stacking faults often occur near the mismatched GaP and AlGaInP interface due to the difficulty in growing GaP layer directly on top of the AlGaInP layer. The long term reliability and stability remain as a big concern in such a light emitting diode because of the generally rough interface structure.